112 ANIMAL LIFE IN THE YOSEMITE 



At Blacks Creek, west of Coulter ville, on the nights of May 10 and 11, 

 1919, a line of 37 traps, set through a mixed stand of brush plants on a 

 shaly hillside, produced 7 and 8 Gilbert Mice. They seemed to be the only 

 mice of the white-footed group {Peromyscus) present there. They were 

 evidently finding daytime shelter in heaps of shale at the mouths of old 

 prospect holes, in weathered outcrops of the same rock, and in tangles of 

 dead brush. 



At El Portal and Pleasant Valley the numbers of this species were less 

 than those of the Gambel and Boyle white-footed mice though greater than 

 those of the Parasitic Mouse. No conspicuous differences in habitat were 

 evident between these several species and it yet remains to work out their 

 ecology. A postulate in animal distribution is that no two species can 

 permanently occupy exactly the same niche in nature, and the evidence 

 in many cases is convincing. But with the several species of white-footed 

 mice there is still much to be explained in this connection. 



One of the Gilbert Mice obtained at Blacks Creek was caught only by 

 the tail and as it seemed uninjured it was carried to a pool in the creek 

 bed to test its swimming abilities. The instant it touched the water the 

 mouse began to swim, using all four feet, and soon gained the bank. It 

 walked slowly up the rocks, but when the observer made an attempt to 

 follow, the mouse put on speed, ran quickly up the hill, and was lost to 

 view in the brush. 



The breeding season of this mouse is not known with any certainty. 

 Between May 25 and June 3, 1915, 5 females, each containing 3 or 4 em- 

 bryos, were trapped. Blue-pelaged juvenals were obtained at about the 

 same time, and suckling females were captured in July. These meager 

 data point to a breeding season three months in extent, with the probability 

 that it is of somewhat longer duration. 



Parasitic White-footed Mouse 

 Peromyscus calif ornicus calif ornicus (Gambel) 



Field characters. — Size more than twice that of House Mouse or of Common White- 

 footed Mouse; ear very large (see fig. 10c) ; tail longer than head and body. Head and 

 body 4 to 5 inches (99-123 mm.), tail 4% to 5% inches (117-136 mm.), hind foot 1 

 to 1% inches (25-28 mm.), ear from crown % to % inch (21-23 mm.); weight iy2-l% 

 ounces (41.5-48.4 grams). General coloration dusky brown on upper surface, sharply 

 set off from pure white of under surface; feet white. 



Occurrence. — Resident in Upper Sonoran Zone on west flank of Sierra Nevada where 

 recorded at Pleasant Valley and El Portal. Lives on hillsides covered with oaks and 

 chaparral; sometimes about deserted nests of Streator Wood Eat. Solitary. 



The Parasitic White-footed Mouse is the largest of our four species of 

 white-footed mice ; indeed, in point of size it approaches an immature 

 wood rat. The name 'parasitic' was applied to this mouse because it is 



